In the printing industry different types of sheet inverters are known, which are typically used in a duplex path of the printer. One type of sheet inverter, which is for example described in U.S. Pat. No. 6,626,103 B1 enables the leading edge of a sheet to remain the same before and after inversion of the sheet. This can be beneficial for registered printing on the front and backside of the sheet.
This known sheet inverter has two transport belts, which are entrained about respective pairs of rollers. The belts are guided in a figure eight configuration around the respective pairs of rollers such that in each free section thereof, extending between the roller pairs, a 180° twist is present. The first and second pairs of rollers are arranged adjacent to each other, such that a section containing the 180° twist of one belt contacts a corresponding section of the other belt, thereby forming a transport path there between, having said 180° twist. The twist forces a sheet, which is received and transported between the belts, to twist along the contact line and to turn by 180°, while it is transported along the transport path. The inverter also has a guide wire for guiding an edge portion of the sheet upwards and across the transport path.
This arrangement is suitable for a wide range of sheets, which can differ with respects to stiffness and dimensions. With respect to long sheets, in particular, sheets having a length (in the direction of transport), which is longer than half the length of the transport path through the sheet inverter, this arrangement can cause problems, as explained herein below. In this case, when the leading edge of the sheet reaches the midpoint of the transport path, both the twist in the transport path and the guide wire urge the sheet into an upright orientation. While the transport path urges only a middle section of the sheet into the upright orientation, the guide wire urges the edge section of the sheet into the upright orientation. When the sheet is longer than half the length of the transport path, the trailing edge will still be held in a horizontal position between the entrance group of rollers at the entrance end of the transport path. This situation can lead to paper jams, undue high stress in the sheet and in particular to artifacts in a printed image on a surface of the sheet due to excessive bending thereof. This problem is obviously more pronounced the longer and stiffer the sheet, which is particularly true for packing material such as cardboard. Also, with wider formats, the guidance of the edge section is problematic.
Further, reference is made to DE 10 2007 025 085 B3, which discloses a sheet inverter for bank notes or bills and the like, which includes two oppositely arranged, contacting transport belts for receiving the bills therebetween. The transport belts are conjointly rotatably supported by a rotating assembly to serve as a sheet inverter.